A crop is a plant cultivated by man for a specific purpose. A plant disease is a deviation of the plant from the normal state of health, presenting outward visible signs. Diseases are caused by pathogens and enhanced by some physiological factors.


Plant diseases are caused by pathogens. Pathogens are disease causing organism which passes through a regular cycle of development and reproduction. Examples of pathogens that cause plant disease are viruses, bacteria, fungi, parasitic worms and rarely protozoa. Some of these pathogens are carried by vectors and other agents.

Physiological factors such as nutrient deficiency in the soil, heat, presence of inorganic salts in the soil and soil moisture content has a major role to play in influencing plant susceptibility to diseases.

Crop diseases can also be caused by various factors, including biological agents, environmental conditions, and cultural practices. Here are some common causes of crop diseases:

1. Pathogens: Pathogens are microscopic organisms such as fungi, bacteria, viruses, and nematodes that can infect crops and cause diseases. They invade plant tissues, disrupt their normal functioning, and lead to various symptoms like wilting, spotting, rotting, and stunted growth.

2. Fungi: Fungal diseases are among the most common crop diseases. Fungi thrive in humid and warm conditions and can attack various parts of the plant, including leaves, stems, fruits, and roots. Examples of fungal diseases include powdery mildew, rust, blight, and damping-off.

3. Bacteria: Bacterial pathogens can infect crops and cause diseases that result in symptoms such as leaf spots, wilting, cankers, and rotting. Bacterial diseases can spread rapidly under favorable conditions, such as high humidity and wounds on the plants.

4. Viruses: Plant viruses are infectious agents that invade plant cells and disrupt their normal functions. They are usually spread by vectors like insects, nematodes, or through infected seeds. Viral diseases can cause various symptoms like mosaic patterns on leaves, stunting, curling, and yellowing.

5. Nematodes: Nematodes are microscopic worm-like organisms that can attack plant roots and cause significant damage. They feed on plant tissues, affecting nutrient uptake and causing stunted growth, wilting, and root galls. Nematode infestations are more common in sandy soils.

6. Environmental conditions: Adverse environmental conditions can weaken crops, making them more susceptible to diseases. Factors such as temperature extremes, excessive rainfall, drought, humidity, and poor air circulation can create favorable conditions for pathogen growth and spread.

7. Poor cultural practices: Improper agricultural practices can contribute to the development and spread of crop diseases. These include practices such as inadequate crop rotation, overuse or misuse of pesticides, improper irrigation, lack of proper sanitation and hygiene, and use of infected seeds or planting material.

8. Genetic susceptibility: Some crop varieties may be inherently more susceptible to certain diseases due to their genetic makeup. Lack of genetic resistance or tolerance to specific pathogens can make crops more prone to infections and diseases.

It is important for farmers to implement integrated pest management strategies, including good agricultural practices, disease-resistant crop varieties, proper sanitation, crop rotation, and judicious use of pesticides, to minimize the risk of crop diseases and protect their harvests.


Name Casual organism Method of transmission Symptoms and economic importance Prevention and control measure
Maize Smut Fungus (Ustilago maydis) Fungus spores deposited

on fruits

(i)   Reduced yield

(ii) Galls on ears, leaves and tarsels which later turn black

(i)    Destroy diseased plant.

(ii)  Use resistant varieties.

(iii)           Seed treatment.

Rice Blight Fungus (Piricularia oryzae) Airborne spores on leaves (i)   Small longitudinal red spots on leaves which turn grey or brown

(ii) Reduced yield

(i)    Use clean seeds

(ii)  Avoid heavy use of nitrogen fertilizers.

(iii)            Use resistant varieties

Maize Rust Fungus (Puccinis polysora) Airborne spores deposited on leaves (i)   Red spots on leave.

(ii) Reduced yield

(iii)        Death of crop.

(i)    Early planting

(ii)  Crop rotation

(iii)           Use resistant varieties.

Cercopora  a Leaf spot of Cowpea Fungus Through


(i)           Reddish brown spots on leaves

(ii)         Lesions on leaves

(iii)       Chlorosis

(iv)        Dropping or falling of leaves.

(i)      Spray with fungicides

(ii)    Crop rotation

(iii)  Plant resistant varieties.

Rosette disease of  Groundnut Virus By piercing and sucking insect (Aphid) (i)      Yellow leaves with mosaic mottling.

(ii)    Stunted plant with curled leaves.

(iii)  Wilting and death of plant.

(iv)   Shortening of the internodes.

(i)      Early planting

(ii)    Crop rotation

(iii)  Use insecticides

(iv)   Uproot and burn infected plants.

(v)     Use resistant variety.

Cassava Mosaic Virus (i)    Through piercing and sucking insect (whitefly)


(Bemisia nigerensis)


(ii)   Infected plant cutting


(i)         Mottling of leaves or leaf curl

(ii)       Distortion of leaves and stems.

(iii)     Vein clearing

(iv)     Stunted growth

(v)        Development of yellowish pale areas alternating with green patches on the leaves or mosaic pattern on the leaves

(i)      Use resistant varieties

(ii)    Uproot and burn infected plant

(iii)  Spray with insecticide to kill vector

(iv)   Use disease-free stem cuttings

(v)      Farm sanitation.

Leaf blight of Cassava Bacterium

Xanthomonas manihotis

(i) Infected     cuttings

(ii)  Rain splashing

(iii)     Insects

(iv)     Tools

(i)              Blighting of leaves

(ii)            Wilting of plant

(iii)          Falling off of leaves

(iv)           Reduced yield

(v)             Cabker of stem

(vi)           Die-back of stem

(i)           Use resistant varieties

(ii)         Use disease free cuttings

(iii)        Early planting

(iv)        Practise crop rotation

Cocoa black pod disease Fungus Phytophthora palmivora (i)  Rain splash

(ii)            Insects

(i)         Brown spots on pod

(ii)       Rottening of pods

(iii)     Entire pod turns black

(iv)      Low yield

(i)           Remove and destroy infected pods

(ii)         Regular weeding

(iii)        Spray with fungicides eg Bordeaux mixture

(iv)        Avoid over crowding of cocoa plants.


Leaf rust

Fungus (i)  By wind

(ii)  By rain splash

(i)       Yellow or brown spot on leaves

(ii)     Orange powdery mass on the leaf

(iii)   Reduction in yield

(iv)   Dropping of leaves

(i)           Plant seeds from healthy plants

(ii)         Use resistant varieties.

(iii)        Spray with copper fungicides.

Black arm (bacterial Blight of cotton) Bacterium (i)    Through leaves

(ii)  Stems near the ground

(i)       Angular spot on leaves

(ii)     Boll rot

(iii)   Exudates from affected leaves

(iv)   Retarded growth and death of plant.

(i)           Seed dressing

(ii)         Uproot and burn infected plants

Root Knot of




Nematodes Nematodes in soil (i)       Knotting or galling of roots

(ii)     Retarded growth


(iii)   Early death of plant

(iv)   Reduction in yield

(i)           Soil sterilization

(ii)         Crop rotation

(iii)        Use resistant varieties

(iv)        Uproot and burn infected plants

Damping off Disease of Okra Fungus Infected soil (i)       Retarded growth

(ii)     Cells become water logged

(iii)   Gradual wilting of plant

(iv)   Death of plant

(i)         Spray with copper fungicide

(ii)       Use resistant varieties

(iii)     Sterilization of soil




Fungus (i)    Infected soil

(ii)  Water splash

(iii)  Infected bulb

(i)       Twisting of leaves

(ii)     Grey patches on leaves

(iii)   Reduction in yield

(iv)   Death of plant

(i)         Crop rotation

(ii)       Use resistant varieties

(iii)     Spray with fungicides

(iv)      Early planting

Stored produce mould fungicides Fungus (i)    Infected seeds or fruits.

(ii)  High humidity

(iii)      By Soil

(i)       Black mould on seeds and fruits

(ii)     Pungent smell.

(iii)   Sour taste

(iv)   Decay of seeds and fruits in store.

(i)           Proper drying of seed before storage

(ii)         Spray with

(iii)        Maintain low humidity in store

(iv)        Remove contaminated seeds before storage.


Diseases can have significant impacts on crop production, leading to reduced yields, lower quality produce, and economic losses for farmers. Here are some of the effects of diseases on crop production:

1. Yield Reduction: Crop diseases can cause a decline in the overall yield of a crop. They can affect various parts of the plant, including leaves, stems, roots, and fruits, leading to reduced growth and productivity. The pathogens responsible for diseases can interfere with essential plant functions such as photosynthesis, nutrient uptake, and water transport, resulting in smaller harvests.

2. Quality Decline: Diseases can also affect the quality of harvested crops. They can cause discoloration, deformities, blemishes, and rotting of fruits, vegetables, or grains. In some cases, the produce may become unmarketable or unfit for human consumption. Crop diseases can also impact the taste, texture, and nutritional value of the harvested crops.

3. Economic Losses: Crop diseases can have severe economic consequences for farmers and agricultural industries. Reduced yields and lower-quality produce can lead to financial losses due to decreased market value and reduced profitability. Farmers may also incur additional costs for disease management, including the purchase of fungicides, pesticides, or other treatments, as well as labor expenses for disease monitoring and control measures.

4. Reduced Food Security: Diseases affecting staple food crops can pose a threat to food security, particularly in regions where agriculture is a primary source of sustenance. When diseases significantly reduce crop yields, it can lead to food shortages, higher food prices, and limited access to nutritious food, potentially impacting the well-being of communities and vulnerable populations.

5. Spread and Long-Term Impact: Crop diseases can spread rapidly, either through direct contact, wind, water, or vectors like insects. If not effectively managed, diseases can persist in the soil or plant debris, leading to recurring infections in subsequent growing seasons. This long-term impact can disrupt crop rotation schedules, limit the choice of suitable crops, and require continuous disease management efforts.

To mitigate the effects of diseases on crop production, various strategies can be employed, including the use of disease-resistant crop varieties, crop rotation, proper sanitation practices, regular scouting and monitoring, and the judicious use of fungicides and other control methods. Early detection, timely intervention, and integrated pest management (IPM) approaches are crucial for minimizing the impact of diseases on crop production.


Diseases can spread on crop farms through various means. Here are some common ways in which diseases can be transmitted:

1. Rain splash: Rainwater can carry disease-causing pathogens and splash them onto nearby crops, spreading the disease.

2. Contaminated equipment: Tools and equipment used on infected crops can carry pathogens and transfer them to healthy plants when used without proper cleaning and sanitation.

3. Infected planting materials: Using infected seeds, seedlings, or other planting materials can introduce diseases to the farm, leading to the spread of the pathogens.

4. Wind transmission: Pathogens can be carried by wind and deposited on neighboring crops, spreading the disease to healthy plants.

5. Animals: Animals such as birds, rodents, and larger mammals can act as carriers of plant diseases. They can transmit pathogens through physical contact, feces, or by feeding on infected plants and then moving to other areas.

6. Insect vectors: Insects like aphids, whiteflies, beetles, and leafhoppers can transmit diseases by feeding on infected plants and subsequently moving to uninfected ones, transferring the pathogens in the process.

7. Visitors to the farm: People visiting the farm, such as farmers, workers, or researchers, can unknowingly carry pathogens on their clothing, shoes, or equipment. This can facilitate the spread of diseases from one area to another.

8. Weeds: Some weed species can act as hosts for plant diseases. If these weeds are present on the farm, they can harbor pathogens and serve as a source of infection for neighboring crops.

It is important for farmers to be aware of these modes of disease transmission and implement appropriate preventive measures such as practicing good sanitation, using disease-free planting materials, controlling weed and pest populations, and regularly inspecting crops for signs of diseases.


Controlling crop plant diseases is essential for ensuring healthy and productive agricultural systems. Here are some common strategies and methods used to manage and control crop plant diseases:

1. Crop Rotation: Crop rotation involves changing the type of crops grown in a particular field from season to season. This practice helps disrupt the life cycle of pathogens that are specific to certain crops, reducing their buildup in the soil and minimizing disease incidence.

2. Resistant Varieties: Plant breeders develop crop varieties that are resistant to specific diseases. These varieties have genetic traits that make them less susceptible to infection or able to tolerate and recover from disease more effectively. Growing resistant varieties can greatly reduce the impact of diseases in agricultural systems.

3. Sanitation and Hygiene: Maintaining good sanitation practices is crucial to prevent the spread of diseases. This includes removing and destroying infected plant material, cleaning and disinfecting equipment, and practicing proper hygiene to minimize the movement of pathogens between fields.

4. Cultural Practices: Several cultural practices can help manage crop diseases. These include proper spacing of plants to promote airflow and reduce humidity, adequate fertilization to maintain plant health and vigor, and appropriate irrigation techniques to avoid excess moisture, which can create favorable conditions for diseases.

5. Integrated Pest Management (IPM): IPM is a holistic approach that combines various pest and disease management strategies. It emphasizes the use of biological controls (e.g., beneficial insects), cultural practices, monitoring and early detection, and judicious use of pesticides as a last resort. IPM aims to minimize the economic, environmental, and health risks associated with pesticide use.

6. Chemical Control: When other strategies are not sufficient, chemical control may be necessary. Fungicides, bactericides, and other pesticides can be used to control specific diseases. However, their use should be based on careful monitoring, accurate diagnosis, and adherence to label instructions to minimize negative impacts on the environment and human health.

7. Genetic Engineering: Advances in genetic engineering techniques allow for the development of crops with enhanced resistance to diseases. Genetic modification can introduce genes from other organisms that confer resistance, providing a valuable tool for disease control. However, the use of genetically modified organisms (GMOs) is a subject of debate and regulation in many countries.

8. Monitoring and Early Detection: Regular monitoring of crops is crucial for early detection of diseases. This allows farmers to take timely action and implement appropriate control measures before the diseases spread and cause significant damage. Monitoring can involve visual inspections, scouting, and the use of diagnostic tools and technologies.

It’s important to note that effective disease management often requires an integrated approach, combining multiple strategies tailored to the specific crop, disease, and local conditions. Farmers, agronomists, plant pathologists, and agricultural extension services play vital roles in implementing these control measures and promoting sustainable disease management practices.


  1. What are pathogens?
  2. List two examples of pathogens.
  3. List three physiological factors that makes plants susceptible to diseases.
  4. List five air borne disease of crops.

See also






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